The invention concerns a printed valuable document with at least one authentication mark in the form of a luminescent substance based on a host lattice doped with at least one rare earth metal,
The protection of valuable documents by means of luminescent substances has long been known. The use of rare earth metals in this connection has also been discussed. These have the advantage of possessing narrow band emission lines, which are particularly characteristic, and can therefore be safely distinguished from the emissions of other substances when using measuring technology Preferably substances are used that have emission lines in the invisible region of the spectrum, particularly in the impaired (IR) spectral region.
In order to enhance protection against counterfeiting still Per, the rare earth metals can be incorporated with other substances into host lattices in such a way that the excitation and/or emission spectrum of the rare earth metal is influenced in a characteristic manner. Through combination with suitable absorptive substances, for example, a part of the excitation and/or emission ranges of the rare earth metal can be suppressed. The influence, however, may also take the form of a xe2x80x9cdistortionxe2x80x9d, e.g. through damping of particular areas of the excitation or emission spectra.
Taking this prior art as a starting point, the objective of the invention is to create a valuable document with an authentication mark in the form of luminescent substances, which in comparison with the prior art are more difficult to detect and therefore offer greater security against counterfeiting.
The fulfillment of this aim is given by the nondependent claims. Further embodiments are the subject of the dependent claims.
As has already been indicated, for examining the authenticity of valuable documents, use is made of the emission lines in the IR spectrum range of the rare earth metals. Preferably, use is made of emission lines which lie in the near IR spectral region, since these can still be detected with economically-priced sensors and because, thanks to the favorable signal-to-noise ratio, measurement errors can be avoided relatively easily. Use is usually made for this purpose of conventional commercial silicon (Si) or germanium (Ge) detectors. The further into the IR spectral region the emission lines lie, the more difficult it is to detect the emission. In very general terns, the principle applies that the detection sensitivity or response sensitivity of photo detectors decreases with increasing wavelength of the radiation to be measured. This means that the signal-to-noise ratio of the detected signals generally becomes smaller as the wavelength increases. As a consequence, the measurement technology for the evaluation of the signals and the expertise required become more and more elaborate. If, in addition, these luminescent substances, which are technically difficult to detect, are only present in small concentrations in the valuable documents to be examined, detection of the emission lines is only possible under special conditions
The invention is based on recognition that the detection of certain specific substances, which becomes difficult with increasing emission wavelength in the IR spectral region, can be used to very great advantage in increasing protection against counterfeiting.
According to the invention, therefore, a luminescent substance is used for safeguarding valuable documents, having an emission spectrum lying outside the response sensitivity of Si or Ge detectors. In this case, recourse must be made, for example, to lead sulphide (PbS), indium arsenide (InAs), gallium-indium-arsenide (Gas), or lead selenide (PbSe) detectors. Their detection sensitivity, however, is less by powers of ten than that of Si detectors. In addition, the evaluation of signals from such a detector with measuring technology is markedly more difficult.
The substances which are suitable for assuring authenticity may be substances based on host lattices doped with holmium. Holmium has emission lines with wavelengths in the region of 2 xcexcm, and therefore can no longer be detected with Ge detectors, since the response sensitivity of Ge detectors in the range from 1900 nm tends towards zero. With the appropriate amount of effort, the substances according to the invention, however, can be detected with the aid of PbS detectors. Since the response sensitivity of PbS detectors in the 2 xcexcm wavelength region is already very low, the holmium must be used in a host lattice which will guarantee the highest possible effectiveness of the holmium, i.e. which will provide for the highest possible quantum yield. According to the invention, host lattices are used containing components which absorb light over a broad band, and transfer the absorbed energy to the holmium with a high degree of efficiency. Preferably, the quantum yield of the luminescent substances according to the invention lies in the range between 50 and 90%.
In addition to this, according to the invention provision is made for the luminescent substances in the individual valuable document to be used in a such a concentration that the properties of the valuable document are not impaired. The maximum concentration depends on a number of different parameters, such as the manner of introduction or the desired properties (colour etc.) of the valuable document.
If the luminescent substance is embedded in a paper pulp, for example, the concentration of foreign substances which is just still permissible is a few percentage points by weight. If the permissible concentration of foreign substances is exceeded, this will result in evident changes to the properties of the material. Too high a concentration of foreign substances in the paper, for example, will reduce the tear resistance of she paper. If the luminescent substance has its own colour, it may be possible that a concentration of about 0.1% by weight will already be sufficient to change the colour of the paper as a whole. An excessive concentration of foreign substances in printing inks makes the colours brittle and reduces their adhesion to the surface of the document. In this case, too, a concentration of just 1% by weight of a coloured luminescent substance may be sufficient to impair the overall colour impression of the printing ink. If this luminescent substance serves simultaneously as an ink pigment, the limit concentration may, on the other hand, only be reached at the maximum physically possible proportion of solids, of about 80% by weight.
According to the invention, the lower limit concentration in the case of colourless or lightly coloured luminescent substances, when mixed into the paper pulp, is 0.1% by eight. With more strongly coloured luminescent substances, the limit concentration may be as low as 0.01% by weight. Preferably, the concentration lies in the range between 0.05 and 1% by weight. The lower limit concentration of the luminescent substance in a layer applied to the valuable document, by contrast, is about 1% by weight, for example for coloured luminescent substances. Depending on the composition of the layer and its purpose, the concentration lies in the range between 1 and 40% by weight, and preferably in the range between 10 and 30% by weight.
The introduction of the luminescent substances in concentrations which are just still permissible, i.e. which do not change the given properties of the valuable document, hinders attempts at counterfeiting whereby, in ignorance of the true luminescence substances, less effective substitute substances with similar emission lines are used; then, however, in order for measurable signals to be obtained, higher concentrations of the substitute substances must be introduced to the valuable document. This leads to identifiable changes in the document or the printing ink containing the luminescent substance. In the case of coloured substances, for example, this would also lead to a discoloration of the valuable document or of the colour of the printing ink.
The luminescent substances can, according to the invention, be applied to the valuable document in a variety of ways. Thus, for example, as already mentioned, the luminescent substances can be mixed into a printing ink which contains additional visible colour additives. Addition of the luminescent substances to the paper pulp is also possible. Likewise, the luminescent substances can also be provided on or in a plastic substrate material, which is at least partly embedded, for example, in the paper pulp. The substrate material in this case can take the form of a security thread, a mottling fiber, or a planchet.
The plastic or paper substrate material can, however, be attached to any other object desired, for the purpose of product security, for example. The substrate material in this case is preferably formed into a label. If the substrate material is a constituent part of the product to be secured, such as is the case with a tear-off thread, for example, any other shape is naturally also possible. In certain specific application instances it may be a good idea for the luminescent substance to be applied as an invisible coating on the valuable document. In this situation, it may be present full-surface or in the form of specific patterns, such as stripes, lines, circles, or even in the form of alphanumeric signs. In order to guarantee the invisibility of the luminescent substance, use must be made according to the invention either of a colourless luminescent substance in one maximum concentration for the printing ink or the coating material, which is still just short of interfering with the properties of the coating, or a coloured luminescent substance is used in such a low concentration that the tenancy of the coating is still just maintained.
The term xe2x80x9cvaluable documentxe2x80x9d in the context of the invention is to be understood to mean bank notes, cheques, shares, stamps, identity cards, credit cards, passes, and other documents, as well as labels, seals, packing, or other elements for protecting products.
The luminescent substances according to the invention feature emission spectra which lie so far into the IR spectrum range that they ran only just be detected with the detectors available for this range with a great deal of technical effort if they are mixed into the valuable document to be marked in the maximum quantity which can be added without changing the document.
Luminescent substances of this nature are designated hereinafter as xe2x80x9climit luminescent substancesxe2x80x9d.
By contrast with other luminescent substances which do not lie at these limits, these substances possess the technical advantage from the security point of view that they are practically unused in other technical fields, and therefore are not commercially available. In addition, the technology required to detect their presence is so elaborate that the risk of analysis of the measurement parameters is relatively low. Moreover, even if the existence of the luminescent substance were known to a forger, he could only replicate it, as already explained, by exactly duplicating all the parameters responsible for the luminescence. Luminescent substances will poorer properties will either create a lasting change in the properties of the valuable document, or will no longer be capable of being detected in the detection devices.